Silicon photonics is a technology for implementing various optical functionalities in silicon and provides a promising solution to address the ever-demanding bandwidth and power-consumption bottlenecks in both on-chip and off-chip interconnections. The active involvement of well-established Complementary-Metal-Oxide-Semiconductor (CMOS) foundries paves the way for custom fabrication processes tailored for large-scale electronics-photonics integrations. One step toward this integration is the development of efficient chip-scale photodetectors (PDs) integrated on silicon, especially PDs operating in the near infrared region.
Near-infrared photodetection may be realized in a variety of materials, including germanium, polycrystalline silicon, III-V materials, and two-dimensional materials. Among them, germanium has the advantages of high responsivity and CMOS compatible integration on silicon. The bandgap of germanium makes it a useful photo detecting material for wavelengths below 1.55 μm.
Conventional methods of sub-bandgap photodetection (i.e., photodetection beyond 1.55 μm) with germanium detectors include increasing the detector size. However, increasing the detector size can induce a larger dark current and result in slower speed performance. Alternatively, bulk germanium can be strained to engineer its band gap so as to perform photodetection beyond 1.55 μm. However, the interaction length with strained germanium is still relatively short, thereby limiting the efficiency of photodetection at longer wavelengths (e.g., in the L band from about 1565 to about 1625 nm).